Damper assemblies which incorporate mechanically actuated or movable valve elements to change the damping coefficient of the assembly are well known and typically operate using conventional hydraulic fluids. In order to improve the performance characteristics of controllable dampers and improve their reliability under harsh or repetitive operating conditions, electroactive fluids having controllable physical properties may be employed in conjunction with the damper to substantially reduce the required number of moving components and operation response time.
Electroactive fluids consist of suspensions of very fine particles in a dielectric liquid media. The most widely used type of electroactive fluids are electrorheological (or "electroviscous") fluids. Electrorheological fluids are electroactive fluids which, in the absence of an electric field, exhibit traditional Newtonian flow characteristics such that their shear rate is directly proportional to shear stress. However, when an electric field on the order of 10.sup.3 V/mm is applied, a yield/stress phenomenon occurs such that no shearing of the fluid takes place until the shear stress exceeds a yield value which rises with increasing electric field strength. The result can appear as an increase in apparent viscosity of several orders of magnitude. Many commercially realizable systems employing electrorheological fluids have bee developed which include variable damper assemblies. See Klass, U.S. Pat. No. 3,207,269; and Stangroom, U.K. Patent No. 2,111,171B. These devices operate by taking advantage of the ability of the electrorheological fluid to, in the presence of an electric field, "thicken" into a semisolid or solid condition where the particles of the fluid form into fibrillated, "pearl-chain" like structures between opposing surfaces of the device. This phenomenon can be utilized to engage relatively movable surfaces of the device for the control of translational motion between members connected thereto. While electrorheological fluid dampers are beneficial in providing for rapid and reversible response characteristics with typical response times being on the order of one millisecond, the force transmission limits of electrorheological fluid devices are constrained by the voltage potential and interactive surface area required to develop a fluid yield strength sufficient for their adequate performance.
Another type of electroactive fluids are electrophoretic (or "electroseparatable") fluids. Electrophoretic fluids are suspensions similar to electrorheological fluids but are characterized by a very different response to an applied electric field. The particles within electrophoretic fluids exhibit a very strong electrophoretic migration. Rather than forming, in the presence of an electric field, a fibrillated structure that has an induced yield strength, electrophoretic fluids separate into particle-rich and particle-deficient phases by electrophoresis. The electrophoretic induced separation can produce much larger yield strengths at lower operating voltages. Electrophoresis is a linear phenomenon with respect to electric field strength, while in contrast, the yield strength of an electrorheological fluid varies with the square of the electric field because of the dependence on induced dipole interactions for the electrorheological effect. Further, once electrophoretic induced separation is accomplished, the resulting yield strength of an electrophoretic fluid can be maintained under a reduced electric field.
Because electrophoretic fluids operate in a substantially different manner from electrorheological fluids in the presence of an electric field, their use in existing electrorheological fluid dampers and other devices would not be functional in many instances. Known electroactive fluid dampers depend on a fibrillated interaction of fluid particles for interconnecting relatively movable surfaces or restricting fluid flow or movement, and are not constructed to take advantage of the yield stress developed by separation of the fluid into particle-rich and particle-deficient phases.
In view of the foregoing, there is a need for a controllable damper which utilizes an electrophoretic fluid in association with its interactive surfaces to provide improved variable damping or braking.